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From: Ian Kelly <ian.g.kelly@gmail.com>
Date: Fri, 29 Jul 2011 17:01:04 -0600
Subject: Re: Identical descriptor value, without leaking memory?
To: Jack Bates <ms419@freezone.co.uk>
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On Mon, Jul 25, 2011 at 11:46 AM, Jack Bates <ms419@freezone.co.uk> wrote:
> How can you get a descriptor to return an identical value, each time
> it's called with the same "instance" - without leaking memory?

class MyDescriptor(object):
    def __get__(self, instance, owner):
        try:
            return instance.__cached_value[self]
        except AttributeError:
            instance.__cached_value = {self: get_value(instance, owner)}
        except KeyError:
            instance.__cached_value[self] = get_value(instance, owner)
        return instance.__cached_value

class MyClass(object):
    my_attribute = MyDescriptor()


Placing the cache on the object instance ensures that it won't outlive
the object, and since the full attribute name will be
_MyDescriptor__cached_value, it's highly unlikely to conflict with an
existing attribute.  Keying the value on self is necessary in case the
object has multiple descriptors of the same type.